Maternal pelvis

MATERNAL

PELVIS

BONY PELVIS

The pelvis is made up of two innominate bones,

sacrum, coccyx.

The pelvis may be divided by linea terminalis

into an upper part known as pelvis major or false

pelvis and a lower part known as pelvis minor or

true pelvis.

PLANES AND DIAMETERS

Pelvis has 4 imaginary planes and each plane has

its own diameters.

1) Plane of pelvic inlet or superior strait

2) Plane of greatest pelvic dimensions

3) Plane of midpelvis or least pelvic dimensions

4) Plane of outlet or inferior strait

TRUE PELVIS Anterior wall and posterior wall of the true pelvis

measures 5 cm and 10 cm respectively.

In the standing position, upper part of pelvic

canal is directed downwards and backwards and

the lower part curves and is directed downwards

and forwards.

The side walls of true pelvis converge a little.

The true pelvis is divided into 3 parts:

Inlet

Cavity

Outlet

PELVIC INLET Boundaries:

Posteriorly by: Sacral promontory and alae of the

sacrum.

Laterally by: Linea terminalis.

Anteriorly by: Horizontal rami of Pubic bones and

pubic symphysis.

Typically round than ovoid.

DIAMETERS :

1. Anteroposterior diameter.

Obstetric conjugate(10.5 cm): shortest distance from symphysis pubis to the middle of sacral promontory.

Most important AP diameter.

Presenting part of Fetus should pass through it.

True conjugate/Anatomic conjugate/Conjugate vera(11 cm): distance from middle of sacral promontory to the superior surface of the pubic symphysis.

Diagonal Conjugate(12.5 cm) extends from

subpubic angle to the middle of sacral

promontory.

Importance:

Can be measured clinically

Subtracting 1.5 cm from this gives obstetric

conjugate.

The inlet is adequate for a normal fetus if

the diagonal conjugate is 12cm or more.

2.Transverse diameter(13.5cm)

o Widest distance between iliopectineal lines

o Widest diameter of the inlet

o Most fetuses engage in transverse or oblique

diameter

3.Oblique diameters(12.5cm)

o Extends from sacroiliac joint of one side to

opposite iliopectineal eminence

4.Posterior saggital diameters(5cm)

o Extends from point of intersection of obstetric

conjugate and transvese diameter to middle of

sacral promontory

5.Sacrocotyloid diameter(9.5cm)

Distance between the midpoint of sacral

promontory and the ipsilateral iliopectineal

eminence

MIDPELVIS

Most important plane of the pelvis at the level of the ischial spines.

Cardinal movements of engagement and internal rotation occur here.

Bounded by 4th & 5th sacral vertebrae,whiteline,ischial spines,sacrospinous ligaments and pubic symphysis .

• Diameters :1. Anteroposterior diameter(11.5cm):

extends from junction of 4th and 5th sacral vertebrae to the lower border of public symphysis.

2. Transverse diameter(10.5 cm):

is between ischial spines.

3. Posterior sagittal diameter(6 cm):

extends from the interspinous diameter to the junction of 4th and 5th sacral vertebrae.

PELVIC OUTLET

Made up of 2 triangles with a common base(a line drawn through 2 ischial tuberosities.)

The anterior triangle has the subpubic angle as the apex and the pubic rami and ischial tuberosities as the sides.

DIAMETERS:

1. Anteroposterior diameter(12 cm):

extends from lower margin of pubic symphysis to the sacrococcygeal junction.

2. Transverse Diameter (10.5 cm):

Distance between inner edges of ischial tuberosity.

3. Posterior sagittal diameter(7 cm):

Extends from middle of transverse diameter to the sacrococcygeal junction.

OTHER DEFINITIONS

Subpubic angle:

Formed by meeting of the two descending pubic rami.

In females, it measures 85-90 degrees.

If the angle is less, transverse diameter of the outlet will

also be less.

Waste space of Morris:

Distance between the pubic symphysis and the edge of a

round disc of diameter 9.3 cm.(Approximating fetal head)

placed under subpubic arch.

Normally subpubic angle is well rounded and the distance

between pubic symphysis and the edge of disc should not

be more than 1 cm.

When it is more than 1 cm, AP diameter of outlet is

reduced.

Axis Of Birth Canal:

Obtained by joining the axes of inlet, cavity

and outlet.

It is a curve with the convexity fitting the

sacral curvature and is called anatomical

pelvic axis or the curve of Carus.

Inclination of Pelvis(assessed in standing

position):

The plane of inlet makes 60 degree with the

horizontal.

Affect engagement of the fetal head.

Obstetric axis:

Course taken by the presenting part as it

moves down through the pelvis.

At first it goes downwards and backwards upto

the level of ischial spine.

Then, the direction changes and at the outlet it

moves downwards and forwards.

CLASSIFICATION OF PELVIS

(BY CALDWELL AND MOLOY)

Based on shape of pelvis:

1. Gynaecoid pelvis Inlet is slightly oval or round(Transverse

diameter equal to or greater than AP diameter)

In the midpelvis, sidewalls of pelvis are straight and the spines are not prominent.

At the outlet, pubic arch is wide.

2. Android Pelvis• At the inlet, posterior sagittal

diam.<Anterior Sagittal diam.

• In the midpelvis, sidewalls are convergent, ischial spines are prominent.

• At the outlet, subpubic arch is narrow.

5. Anthropoid Pelvis

• At the inlet, AP diam.>Transverse diam.

• In the midpelvis, sidewalls are

convergent, ischial spines are likely to be

prominent.

• At the outlet, subpubic arch is narrowed,

but well shaped.

4. Platypelloid pelvis:

• Flattened gynaecoid pelvis with a short

AP and a wide transverse diameter.